Could be once a week, once every few days, or even once every two weeks.
(Studies found no difference in disease prevention for vitamin D being taken daily or monthly)

Example: say you wanted to average 4,000 IU/day.

Could just take a 50,000 IU dose every 12 days - rounded up to being 2 weeks to make it easy to mark on the calendar
When you have taken the supplement, just strike a line thru the D or the dot

Note from Henry Lahore - admin of VitaminDWiki

I had struggled with remembering to take supplements for decades – and still have not gotten into the habit.
I had always failed to remember to take supplements on a daily basis.
I now take a 50,000 IU dose every 5 days (10,000 IU/day average)

Here is what D days might look like on a variety of monthly calendars

Similarly: could mark D days on paper calendars which are daily, weekly or annual; as well as electronic calendars

See also VitaminDWiki

• 15 minutes of education improved vitamin D compliance – Mar 2012
• After learning they were vitamin D deficient, only 8 percent normalized – April 2012
• How often should I take 50000 IU of vitamin D
• Harris 2003 survey of problems swallowing pills
  40% of American adults have experienced difficulty swallowing pills
  20% have hesitated prior to taking pills because they thought they might have trouble swallowing them.
  About twice as many women (51%) as men (27%) experienced pill-swallowing problems.
     Note: vitamin D pills are very small, can be chewed
        vitamin D is also available in other forms such as drops and microdots
• How you might double the benefit of your vitamin D
  Take vitamin D with the largest meal of the day 56%, or evening meal
  Have far more mono-unsaturated fats in your diet than poly-unsaturated fats
     I suggest taking vitamin D with a few almonds
• Vitamin D prior to and after birth annual calendar
  Includes increasing to provide enough vitamin D while breast feeding - without any vitamin D supplements for the infant.
• People reported taking pills, pills were all gone, but occasionally bottles were not opened – Dec 2015